1. Field of the Invention
This invention relates to a heat treatment process which can improve certain properties (in particular, ductility) of a Ni-base heat-resisting alloy used as a material for high-temperature components such as stationary blades of gas turbines.
2. Description of the Related Art
Ni-base heat-resisting alloys, which combine precipitation strengthening by .gamma.' phase (Ni.sub.3 (Al,Ti,Nb,Ta)) with solid solution strengthening by Mo, W or the like, are being used for high-temperature components such as stationary blades of gas turbines. For these Ni-base heat-resisting alloys, attempts have been made to improve their properties such as high-temperature strength, corrosion resistance and weldability, by controlling the state of precipitation of .gamma.' phase, for example, through adjustment of the proportions of constituent elements or through the addition of very small amounts of certain elements. Although such attempts are effective in improving the respective properties, it is difficult in the present situation to obtain a Ni-base heat-resisting alloy having a well-balanced overall combination of good properties.
When attention is paid to high-temperature strength and weldability among various properties, it is generally known that an increase in the amount of .gamma.' phase precipitated causes an improvement in high-temperature strength, but tends to reduce weldability. For example, an alloy in which the amount of .gamma.' phase precipitated is increased to improve high-temperature strength (Japanese Patent Publication (JP-B) No. 54-6968/'79) has poor weldability, and an alloy in which the amount of .gamma.' phase precipitated is decreased to improve weldability (Japanese Patent Provisional Publication (JP-A) No. 1-104738/'89) shows a marked reduction in high-temperature strength.
As a Ni-base heat-resisting alloy having improved weldability without detracting from its high-temperature strength, the present inventors have previously developed and proposed a Ni-base heat-resisting alloy containing, on a weight percentage basis, 0.05 to 0.25% C, 18 to 25% Cr, 15 to 25% Co, 5 to 10% (W+1/2Mo) (provided that (W+1/2Mo) comprises one or both of 0 to 3.5% Mo and 5 to 10% W), 1 to 5% Ti, 1 to 4% Al, 0.5 to 4.5% Ta, 0.2 to 3% Nb, 0.005 to 0.1% Zr, and 0.001 to 0.01% B, the balance being Ni and incidental impurities, and having a composition defined by the fact that, on the graph of FIG. 1 plotting the weight percentage of (W+1/2Mo) as ordinate and the weight percentage of (Al+Ti) as abscissa, the (Al+Ti) content and the (W+1/2Mo) content fall within the range enclosed by the straight lines connecting point A [3% (Al+Ti), 10% (W+1/2Mo)], point B [5% (Al+Ti), 7.5% (W+1/2Mo)], point C [5% (Al+Ti), 5% (W+1/2Mo)], point D [7% (Al+Ti), 5% (W+1/2Mo)] and point E [7% (Al+Ti), 10% (W+1/2Mo)] in the order mentioned (Japanese Patent Provisional Publication (JP-A) No. 8-127833/'96). This Ni-base heat-resisting alloy will hereinafter be referred to as alloy A.
Although the above-described alloy A is a Ni-base heat-resisting alloy having excellent high-temperature strength and weldability, attention paid to high-temperature ductility reveals that the balance between high-temperature strength and high-temperature ductility is not satisfactory. When alloy A is subjected to a tension test, for example, at 850.degree. C., it shows an elongation of as low as 5% or so because a fracture readily occurs at grain boundaries.
It is generally known that high-temperature ductility affects thermal cycle fatigue strength at elevated temperatures. Accordingly, it is desirable that components requiring excellent thermal cycle fatigue strength, such as stationary blades of gas turbines, show an elongation of not less than 8% in a tension test at 850.degree. C.